Patch "rcu: Add memory barriers for NOCB leader wakeup" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

rcu: Add memory barriers for NOCB leader wakeup

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
rcu-add-memory-barriers-for-nocb-leader-wakeup.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Paul E. McKenney" <[email protected]>
Date: Fri, 28 Apr 2017 20:11:09 -0700
Subject: rcu: Add memory barriers for NOCB leader wakeup

From: Paul E. McKenney <[email protected]>

commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0 upstream.

Wait/wakeup operations do not guarantee ordering on their own. Instead, either locking or memory barriers are required. This commit therefore adds memory barriers to wake_nocb_leader() and nocb_leader_wait().

---
 kernel/rcu/tree_plugin.h |    2 ++
 1 file changed, 2 insertions(+)

@@ -1767,6 +1767,7 @@ static void wake_nocb_leader(struct rcu_
 	if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
 		/* Prior smp_mb__after_atomic() orders against prior enqueue. */
 		WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
+		smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
 		swake_up(&rdp_leader->nocb_wq);
 	}
 }

@@ -2021,6 +2022,7 @@ wait_again:
 	 * nocb_gp_head, where they await a grace period.
 	 */
 	gotcbs = false;
+	smp_mb(); /* wakeup before ->nocb_head reads. */
 	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
 		rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
 		if (!rdp->nocb_gp_head)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/rcu-add-memory-barriers-for-nocb-leader-wakeup.patch